Thanos. He has experience dealing with powerful telepaths and that's all Overmind has going for him.
Overmind's best and worst showings were in the Quasar "Journey into Mystery" arc. By best I mean he took the Stranger by surprise and was in a prolonged psi war with him and looked like he was winning. By worst I mean he couldn't even deal with Quasar (because his Q-bands protected him but barring that why didn't he fry his grey matter with a psi bolt? The Bands only protect you from psionic take over, nothing else).
I feel Overmind is Thanos of his universe, I feel marvel miss the boat on him big time, a powerful Eternal with great mental powers plus gladiator/warrior attitude. They can still use him, get the right writer Overmind can be a top notch villain that would eventually give Thanos a run for his money.